In classical silver halide photography a photographic element, commonly referred to as a taking film, containing at least one silver halide emulsion layer coated on a transparent film support is imagewise exposed to light, producing a latent image within the emulsion layer. The film is then photographically processed to transform the latent image into a silver or dye image that is a negative image of the subject photographed. The resulting processed photographic element, commonly referred to as a negative, is placed between a uniform exposure light source and a second photographic element, commonly referred to as a photographic paper, containing at least one silver halide emulsion layer coated on a white paper support. Exposure of the emulsion layer of the photographic paper through the negative produces a latent image in the photographic paper that is a positive image of the subject originally photographed. Photographic processing of the photographic paper produces a positive of the subject image. The image bearing photographic paper is commonly referred to as a print.
While both negatives and prints rely on radiation-sensitive silver halide emulsions for image capture, the choices of emulsions for these separate applications are quite different. Silver halide emulsions used in photographic paper to produce prints are usually subjected to high intensity, short duration exposures from a controlled light source. The silver halide emulsions chosen for prints typically contain high (&gt;50M % and, more typically, &gt;90M %) proportions of silver chloride and low (&lt;5M % and, more typically, &lt;1M %) silver iodide to facilitate rapid processing. As compared to the silver halide emulsions employed in taking films, the speeds of silver halide emulsions used to form prints are limited. Speed limitations can be tolerated, since the light source for exposure is entirely under control.
The silver halide emulsions employed in taking films are usually chosen to realize under available lighting conditions the highest attainable speeds compatible with image quality (e.g., granularity) requirements. To maximize speed and speed in relation to granularity, taking films almost universally employ silver iodobromide emulsions.
Since the early 1980's the very highest speed taking films have increasingly relied upon tabular grain emulsions. These emulsions provide superior speed-granularity relationships at moderate to high photographic speeds. However, nontabular grain silver iodobromide emulsions have remained the emulsions of choice for most taking films used to meet image quality (e.g., low granularity) requirements dictating mean grain ECD's to 0.6 .mu.m or less.
A wide variety of dopants have been employed to modify the properties of the silver halide emulsions. A summary of silver halide grain dopants is included in Research Disclosure, Vol. 365, Sep. 1994, Item 36544, Section I. Emulsion grains and their preparation, D. Grain modifying conditions and adjustments, sub-paragraphs (3), (4) and (5). In looking through dopant selections for silver halide emulsions it is apparent that grain halide content, shape, size and intended modes of exposure and processing all influence dopant selections. Dopant selections are in most instances carefully tailored to serve specific photographic applications.
Kuno U.S. Pat. No. 5,051,344 discloses silver iodobromide emulsions containing 0.1 to 4 mole percent iodide and, as grain dopants, 5.times.10.sup.-9 to 1.times.10.sup.-6 mole of an iridium compound and 5.times.10.sup.-9 to 1.times.10.sup.-6 mole of an iron compound per mole of silver. The grains are of a core-shell structure with the core containing a higher iodide content (at least 3 mole percent greater) than the shell. Kuno specifically prefers both the iridium and iron to be present in the shell. The object is to achieve high contrast with high-illuminance short-duration exposure, rapid processing, and better safe-light handling. The latter requirement, better safe-light handling, is a requirement of increased low intensity reciprocity failure. In other words, the emulsions are intended to be responsive to high intensity, short duration exposures, but relatively unresponsive to the low levels of illumination provided by safe-lights. Kuno recognizes that iridium reduces both high and low intensity reciprocity failure. Kuno's purpose in adding iron is to eliminate the effect of iridium in reducing low intensity reciprocity failure.